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Science: A Sun-like star shows its spots

By
KEN CROSWELL in
BERKELEY

A close neighbour of the Sun, long thought to be a similar kind of star,
may be even more like the Sun than everyone assumed. Astronomers in Canada
and the US have found tentative evidence that Tau Ceti has an 11-year cycle
during which the number of its starspots waxes and wanes, just like the
sunspot cycle.

Tau Ceti is only 11.4 light years away in the constellation of Cetus.
Like the Sun, it is bright, yellow, and a single star. It is so much like
the Sun that in 1960 radio astronomers chose it as their first target
in the search for extraterrestrial intelligence.

David Gray of the University of Western Ontario in London, Ontario,
and Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics
in Cambridge, Massachusetts, analysed observations of Tau Ceti which they
and other astronomers had made between 1968 and 1991. These recorded the
light emitted by singly ionised calcium. The strength of this emission
reflects starspot activity.

Like the Sun, Tau Ceti has only weak calcium emissions, indicating that
it too has minimal starspot activity. But Gray and Baliunas found a hint
of an 11-year cycle in their data because the star’s calcium emission was
strongest in 1980 and 1991. This suggests that Tau Ceti has a star-spot
cycle that peaked in those years. The Sun behaves in a similar way, with
the number of sunspots increasing and decreasing every 11 years.

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Many other stars that resemble the Sun also have starspot cycles, but
the astronomers say that they need more data before they can be sure that
Tau Ceti has an 11-year cycle. Part of the problem is that the spot activity
is weak, making the detection of variations difficult. Another problem
is that the star’s polar axis points nearly at Earth. This is the worst
possible orientation for studying Tau Ceti’s spots if they, like the Sun’s,
appear nearer its equator than its poles.

Tau Ceti is not, however, exactly like the Sun. It has only 40 per cent
of the Sun’s luminosity and only 30 per cent of the Sun’s abundance of heavy
elements, such as carbon, nitrogen, oxygen and iron. This latter feature
is upsetting for those who hope that any planets around Tau Ceti support
life, because Earth-like planets and terrestrial life are made of heavy
elements.

Gray and Baliunas will publish their work in the 1 June issue of The
Astrophysical Journal.